Nowadays, while high compression of image data can be achieved by image data encoding techniques, the pre-processing and post-processing of image data appear to be ever more important. Many techniques have been proposed for filtering noise generated when image data is acquired and encoded.
As shown in FIG. 1, U.S. Pat. No. 5,818,964 discloses a method and an apparatus for selecting an adaptive filter for image data. This prior art has been adopted by the Motion Picture Experts Group-4 (MPEG-4) standard to eliminate associated ringing noise. The noise filtering process 1 of this prior art consists of two sub-processes, that is, an index value generating unit 2 and a selective local smoothing unit 3. The index value generating unit 2 has a threshold value determining unit 4 and a binary index value unit 5. The selective local smoothing unit 3 has a filter selecting unit 6 and an adaptive filtering unit 7. First, an input image is inputted into the index value generating unit 2. Subsequently, the threshold value determining unit 4 calculates the maximum and minimum gray levels for every block consisting of 8×8 pixels in the input image to determine a threshold value. In the binary index value unit 5, the threshold value is compared with each pixel value in a corresponding block so as to generate binary indices. In the filter selecting unit 6, the binary indices are used as a basis to select a smoothing filter. The adaptive filtering unit 7 is for performing a smoothing filter process so as to adjust the gray level of a block.
However, since the abovementioned prior art uses the maximum and minimum pixel values in a corresponding block to determine the threshold value, textures and edges that only present local variations cannot be effectively detected. Therefore, the textures and the edges are likely to be classified as noise and smoothed, thereby reducing the sharpness in images. In addition, real noise may possibly be classified as textures or edges and thus not be removed. Therefore, a solution is necessary.